Among the pharmacologically important Class B G protein-coupled receptors (GPCRs), the PAC1/VPAC receptors (ADCYAP1R1/VIPR1/VIPR2) for pituitary adenylate cyclase activated polypeptide (PACAP, ADCYAP1) and vasoactive intestinal peptide (VIP) have been implicated in several disorders, including chronic pain and stress-related behavioral abnormalities which are among the most prevalent global neurological challenges today. However, the development of reagents targeting these receptors for potential therapeutics has been hampered by the lack of full-length PAC1/VPAC receptor structural information, and mechanistic understandings of how ligand binding can differentially drive conformational changes for receptor activation and biased signaling. Accordingly, the overarching aim of this proposal is to employ state-of-the-art computational methods to model PAC1/VPAC receptor structure and dynamics, with the goal of developing small-molecule compounds to modulate their functions. For each receptor subtype, the identification of unique structural features that allow for specific ligand interactions and transducer protein associations will facilitate the rational design and optimization of small-molecule ligands. Towards that goal, three specific aims will be pursued: (1) to model and compare structures and mechanisms that determine neuropeptide selectivity and function among the physiologically relevant PAC1Null, PAC1Hop1, VPAC1, and VPAC2 receptor subtypes; (2) to delineate PAC1/VPAC receptor-transducer protein interactions and specificity; and (3) to develop and optimize small molecules for selective PAC1 and VPAC receptor regulation. Our preliminary data have established the modeling methodology with the PAC1Null receptor, demonstrated distinct signaling behaviors of the PAC1/VPAC receptors, and identified two different PAC1Null antagonists. Under Aims 1 and 2, we will use homology modeling, protein structure refinement, and molecular dynamics simulations to study the receptors that are ligand-free and complexed with a neuropeptide (PACAP/VIP) or a transducer protein (Gs/Gq/?-arrestins). In particular, we will elucidate the binding sites that are key for ligand specificity as well as conformational states that facilitate long-term signaling. Receptor mutagenesis and constructs will inform and/or substantiate the receptor models. Under Aim 3, we will integrate molecular docking and simulations, organic synthesis, and molecular and cellular assays to develop selective small-molecule modulators. Especially, the strategies to target the orthosteric and allosteric sites will be tested. Our multidisciplinary approach is innovative as it provides an unparalleled and comprehensive means to investigate the PAC1/VPAC receptors in various conditions and functional states. Further, the proposed research is significant, because it will close fundamental gaps in understanding how molecular structures and dynamics can dictate PAC1/VPAC receptor mechanisms. Finally, given current limitations in therapeutics, these studies may offer new opportunities and approaches to treat challenging neurological disorders.